Vapour compression systems, such as refrigeration systems, air condition systems or heat pumps, normally comprise at least one compressor, at least one condenser, at least one expansion device, e.g. in the form of expansion valves, and at least one evaporator arranged along a refrigerant path. Refrigerant circulates the refrigerant path and is alternatingly expanded and compressed, and heat exchange takes place in the condensers and the evaporators. Expanded refrigerant enters the evaporators in a mixed state of gaseous and liquid refrigerant. As the refrigerant passes through the evaporators, it evaporates while exchanging heat with a secondary fluid flow, such as an air flow, across the evaporator. In order to utilise the potential refrigeration capacity of a given evaporator to a maximum extent, it is desirable that liquid refrigerant is present along the entire length of the evaporator. On the other hand, it is undesirable that liquid refrigerant passes through the evaporator and into the suction line, since it may cause damage to the compressors if liquid refrigerant reaches the compressors. It is therefore desirable to control the supply of refrigerant to the evaporators in such a manner that, in a given evaporator, the boundary between mixed phase refrigerant and gaseous refrigerant is exactly at the outlet of the evaporator. This is often done by controlling an opening degree of the expansion device, based on one or more measured parameters.
EP 2 894 421 A1 discloses a method for controlling a supply of refrigerant to an evaporator on the basis of measurements provided by two temperature sensors. The opening degree of the expansion valve is set to a calculated opening degree, based on an air temperature of a secondary air flow across the evaporator, overlaid with a perturbation signal. The temperature of refrigerant leaving the evaporator is monitored and the temperature signal is analysed. If the analysis reveals that a dry zone in the evaporator approaches a minimum length, the opening degree of the expansion valve is decreased. This provides a safety mechanism which ensures that liquid refrigerant is prevented from passing through the evaporator. EP 2 894 421 A1 only describes control of the vapour compression system during normal operation, and is not concerned with how the vapour compression is controlled in a contingency mode, i.e. in the case that one or more of the temperature sensors is unavailable or unreliable.
In some prior art methods, an average opening degree of the expansion valve, e.g. during an immediately previous time interval, is calculated in the case that it is established that a temperature sensor, which is normally used for controlling the opening degree of the expansion valve, is unavailable or unreliable. The opening degree of the expansion valve is then set to a reduced opening degree, being an appropriate percentage, e.g. 80%, of the calculated average opening degree. The opening degree of the expansion valve is then maintained at this reduced opening degree until the unavailable or unreliable sensor has been repaired or replaced. This allows the vapour compression system to continue operation, despite the unavailable or unreliable sensor, and the reduced opening degree provides a safety margin which prevents liquid refrigerant from passing through the evaporator. However, the vapour compression system is not operated in an efficient manner, and it is not possible to take possible changes in the refrigeration load, e.g. due to new goods being added to a display case of the vapour compression system, into account.